Dynamic characteristics of tire slip angle and lateral force (see FIG. 1) are expressed in terms of a parameter called relaxation length. The relaxation length varies according to various factors such as ground contact load, and longer delays occur in the generation of lateral force as the relaxation length gets longer, with this adversely affecting the handling performance.
Conventionally, in order to compensate for the influence of relaxation length, consideration is given to the influence of delay in cornering force or self-aligning torque on the power steering performance, and influence of the relaxation length is compensated for by phase lead control for such delay (see JP-A No. 2008-114687).
In the technology described in JP-A No. 2008-114687, phase lead control is performed on SAT compensation values as estimated values of self-aligning torque, and compensation for phase delay is performed.